The malarial 35 kb circular DNA molecule central to this invention corresponds to a minor species of DNA distinct from nuclear DNA discovered in the 1960s (Gutteridge et al. 1971). In the mid-80s the first study on its purification and molecular analysis was published (Williamson et al. 1985). Its similarity was noted to a circular DNA in the related organism Toxoplasma gondii--a well known opportunistic pathogen in AIDS cases.
It is important to stress that the malaria parasite and related apicomplexans are unusual amongst non-photosynthetic organisms in that they possess two forms of organellar DNA, typically a property of plants. One form of organellar DNA has been identified as mitochondrial DNA (mtDNA), whereas the other, the 35 kb circle, we have proposed is the remnant of a plastid DNA (plDNA), a provenance hitherto unsuspected for these organisms (Wilson et al. 1991, 94). This plDNA was probably acquired by an ancient progenitor of the phylum and may be of algal origin (Williamson et al. 1994). The precise location of these organellar DNAs in the cell shows they are in separate compartments (Kohler et al 1997).
Thus, there are potentially two organellar protein synthesising systems of independent prokaryotic origin within the malaria organism that could be susceptible to inhibition with antibiotics. Although the malarial mitochondrion is the best characterised of the organelles, its genetic content is highly idiosyncratic, contributing only incomplete fragments of two rRNA genes to the machinery required for protein synthesis. The circular DNA from the putative plastid, on the other hand, is much more conventional, producing transcripts of four complete rRNA genes, some twenty tRNA genes, subunits of a typical plastid RNA polymerase, and a number of ribosomal protein genes organised in modified bacterial operons.